To achieve high efficiencies in mass transfer columns (e.g., air separation columns), it is well established that uniform liquid distribution in a packing bed is critical. Uniform liquid distribution leads to efficient mass transfer in the packing bed. Thus, it became the industry standard to design mass transfer columns and devices to promote uniform liquid distribution.
For exemplary purposes, a traditional mass transfer column section 100 is illustrated in FIG. 1 that uses countercurrent flow of a liquid and a vapor for mass transfer. The liquid falls down the mass transfer column as a result of gravity and the vapor rises up the mass transfer column as a result of an established pressure gradient along the length of the column section. The result is that mass transfer takes place inside the column.
A typical mass transfer column, such as an air separation column, is divided into a number of zones or sections 102 where each zone or section 102 is bounded by a mass transfer device such as, for example, a packing bed or packing 104, from the bottom and a liquid distributor 110, for example, from the top. Between the packing 104 and the distributor 110 is a space or spacing 108 between the bottom surface 126 of the liquid distributor 110 and the top surface 106 of the packing 104 where vapor 120 ascends upward from the packing 104 and liquid 116 falls freely downward from the liquid distributor 110.
A typical liquid distributor 110 contains both vapor and liquid passages for vapor and liquid collection and distribution. The vapor passages are used for the ascending vapor 120 to pass through the liquid distributor 110 into the next column section (not shown). Liquid collectors (not shown) are located on the top of the liquid distributor 110. The liquid collector and liquid distributor 110 are typically designed to maintain a desired level of liquid 116 and to provide a desired, usually even, liquid distribution across the surface of the liquid distributor 110 and, therefore, across the column cross-sectional area. The purpose of the liquid distributor 110 is to distribute the liquid 116 uniformly on the packing surface 106. A series of liquid distribution apertures or holes 114 are placed in the liquid distributor 110 for the liquid 116 to pass through under hydrostatic pressure. The liquid distribution apertures or holes 114 may be of equal or different diameters depending on the mass transfer column size, specific zone or section design, position on the liquid distributor surface, etc. In addition, the liquid distribution apertures or holes 114 may be organized in regular or irregular arrays. The liquid distribution apertures or holes 114 may be placed at the bottom of the distributor body or at the trough vertical walls, etc.
Liquid streams of droplets 118 form after passing through the liquid distribution apertures or holes 114 of the liquid distributor 110 and the streams of droplets 118 fall from the liquid distributor 110 through the spacing 108 creating streams of droplets 118 or liquid streams. In general, the liquid streams or droplets 118 created may vary in size and may have different initial velocities. The droplet sizes are defined by the diameter of the liquid distribution apertures or holes 114, by the liquid initial velocity, and by the liquid physical properties (density, viscosity, etc.). The liquid initial velocity is defined by the number of liquid distribution apertures or holes 114, the diameter of the liquid distribution apertures or holes 114, and the level of the liquid 116 above the liquid distributor 110. The droplets 118 fall down freely against the ascending vapor 120 in the spacing 108.
The packing or packing bed 104 is designed to accept the liquid 116 from the liquid distributor 110 and to distribute the ascending vapor stream 120 evenly across the column cross section 102. Therefore, one may assume that the ascending vapor 120 ascends evenly up to the liquid distributor 110 where it shall split into a series of streams penetrating the open areas, called riser areas, riser apertures, or risers 112 hereinafter, organized across the surface of the liquid distributor 110. Riser walls 113 prevent liquid collected in the liquid distributor 110 from flowing downwardly through the risers 112.
The split of the ascending vapor 120 into a series of streams may not be uniform, however, and depends primarily on the open area, geometry, and position of the risers 112. As illustrated in FIG. 2, as the vapor streams 120 ascend through the spacing 108, the vapor streams 120 begin to accelerate and turn towards the open area of the risers 112 where the vapor streams 120 may escape into the next column section, for example. These turning vapor streams 122 create a force directed towards the center of the open area of the risers 112 and on the falling liquid streams or droplets 118. In the traditional column section 102, the falling liquid or droplet 118 may experience the impact of the force produced by the ascending turning vapor streams 122 the moment the droplet 118 exits the liquid distributor 110 into the spacing 108. The interaction between turning vapor stream 122 and the falling droplets 118 influences the intended trajectory of the falling droplets 118 (i.e., through deflection of the falling droplets 118). Any significant change from the intended trajectory of the falling droplets 118, and thus, the droplets' 118 intended target(s) on the packing surface 106, may lead to maldistribution and poor performance of the mass transfer column section 100.
The force acting on the droplets may be different in the vicinity of different risers since the vapor stream may split differently as mentioned above. Typically, the ascending vapor begins its split into different streams in the space between the top of the packing surface and the underside of the liquid distributor (i.e. the spacing). A droplet trajectory will depend on the droplet mass, its initial velocity, the position of the liquid distribution apertures or holes relative to the riser edge, and the droplet affected residence time (i.e., the time when the droplet 118 is in the spacing 108 under the influence of the force from the turning vapor stream 122).
Significant deflection of the droplet may occur from a desired fall position if the droplet is experiencing a force from the turning vapor stream at the very top of the spacing and/or if the droplet forms at the liquid distribution aperture or hole positioned close to the riser edge. Further, as vapor and liquid throughput is increased in the mass transfer column, liquid droplet deflection will increase.
There are several ways to minimize liquid deflection in the mass transfer column. The first way to minimize liquid deflection in the mass transfer column is to minimize the spacing 108 between the bottom surface 126 of the liquid distributor 110 and the top surface 106 of the packing 104. Having a smaller spacing results in a shorter affected droplet residence time (ADRT) of the falling liquid in that spacing and, therefore, may result in less overall liquid deflection from the desired fall position at the packing surface. The affected droplet residence time (ADRT) is calculated by dividing the spacing where the droplets are affected by the turning vapors streams (HAFFECTED) by the average droplet velocity (VAVEDROPLET) or:ADRT=HAFFECTED/VAVEDROPLET.Unfortunately, minimizing the spacing often has limits due to a variety of different factors related to fabrication of the mass transfer devices and liquid distributors.
A second way to minimize liquid deflection in the mass transfer column is to reduce the vapor flow rate. This option may greatly impact the force deflecting the liquid streams as shown in FIG. 3, however, this option may not be desirable, especially when products of separation are of great demand and the mass transfer column is forced to operate at its maximum capacity.
A third way to minimize liquid deflection in the mass transfer column is to reduce the ascending vapor velocity inside the riser by increasing the riser open area while keeping the vapor flow rate constant. This approach utilizes less space for horizontal liquid flow resulting in narrower liquid troughs with higher liquid velocities in the troughs and, therefore, may impact liquid distribution at the top of the liquid distributor. Poor liquid distribution in the distributor troughs will make liquid maldistribution even worse at the packing surface. In addition, narrower troughs may demand positioning the rows of liquid distribution apertures or holes closer to the edges of the riser. Such positioning of the liquid distribution apertures or holes may lead to an increased liquid stream deflection and, therefore, may result in even greater liquid maldistribution on the packing surface.
A fourth way to minimize liquid deflection in the mass transfer column is to increase the droplet size of the liquid and to increase the liquid droplet initial velocity. These two approaches are interdependent. Indeed, an increase in the droplet size requires larger diameter liquid distribution apertures or holes, which by itself may reduce the liquid level above the liquid distributor, thereby leading to reduced initial droplet velocity in the spacing. While it is possible to increase the size of the liquid distribution apertures or holes and keep the liquid level above the liquid distributor constant by reducing the number of liquid distribution apertures or holes 114 on the liquid distributor 110, this may lead to a liquid distributor design with too few aperture or holes 114, which by itself may impact the uniformity of liquid distribution on the packing surface and the overall efficiency of the mass transfer column. A simple increase in the liquid level to increase initial liquid velocity may influence the column design (i.e., forcing an increase in column height). Typically, this option is also undesirable in most cases.
Traditional liquid distributor designs may be found in, for example, the following publications: U.S. Pat. Nos. 6,293,526; 6,059,272; 6,395,139; 5,785,900; 5,132,055; 5,868,970; 6,086,055; EP 0972551; and WO 02/083260.
Disclosure and discussion of the problems associated with uniform liquid distribution on the mass transfer device surface or so called packing surface are somewhat limited. This may be because of the well-known, but incorrect, assumption or belief that if a liquid is distributed uniformly at the point where liquid leaves the liquid distributor, the liquid will be distributed uniformly on the surface of a mass transfer device or packing. It is customary to assume that the uniform distribution of the holes on the liquid distributor provides the same uniform liquid distribution at the mass transfer device surface, (i.e., where the liquid entered the packing surface).
For example, disclosure of different aperture dimensions and their suggested positions can be found in prior publications. For example, U.S. Pat. No. 6,293,526 suggests an arrangement of screens placed in front of the liquid distribution apertures through which liquid discharges from the distributor. The screen design was suggested for the distributors with side liquid jets. Liquid jets impinge on the screens, or so-called baffles, and form a liquid layer of the jetted liquid on the baffle surfaces. The liquid layer slides down the baffle surface and drips onto the packing surface positioned below. U.S. Pat. No. 6,293,526 proposes a better baffle inclination angle that minimizes both liquid splashing and the amount of liquid droplets that may be carried upwards in the ascending vapor.
The ends of the suggested baffles of U.S. Pat. No. 6,293,526 are located in the vapor stagnation zone, thus, the baffle length is short and does not protrude far enough through the spacing between the distributor and the packing surface. The baffles of U.S. Pat. No. 6,293,526 are short because the inventors were only motivated to teach use of the suggested baffles to minimize the liquid splashing and formation of the thin liquid layer of the jetted liquid on the baffle surface and not to minimize liquid deflection as disclosed herein.
Thus, there is a need in the art for an improved column section design that permits sustaining performance in a mass transfer column at high production rates by minimizing liquid maldistribution on the packing surface and associated method of use. Such methods and design shall prevent droplet deflection in the spaces between liquid distributors and the packing surfaces for different column zones.